The present invention relates to a chiral smectic liquid crystal device for use in light-valves for flat-panel displays, projection displays, printers, etc., and liquid crystal apparatus using the liquid crystal device or a display panel.
As a type of nematic liquid crystal display device used heretofore, there has been known an active matrix-type liquid crystal device, wherein each pixel is provided with an active element (e.g., a thin film transistor (TFT)).
As a nematic liquid crystal material used for such an active matrix-type liquid crystal device using a TFT, twisted nematic (TN) liquid crystal, as disclosed by M. Schadt and W. Helfrich, Appl. Phys. Lett., vol. 18, no.4, pp. 127-128 (1971), has been widely used.
In recent years, there has been proposed a liquid crystal device of In-Plane Switching mode utilizing an electric field applied in a longitudinal direction of the device or of Vertical Alignment mode, thus improving a viewing angle characteristic which is poor in conventional liquid crystal displays.
Accordingly, there are various liquid crystal modes suitable for the TFT-type liquid crystal device using nematic liquid crystal material. In any mode, however, the resultant nematic liquid crystal display device has encountered a problematically slow response speed of several ten milliseconds or more.
In order to improve the response characteristic of conventional types of nematic liquid crystal devices, several liquid crystal devices using a specific chiral smectic liquid crystal, such as a ferroelectric liquid crystal of a short pitch-type, a polymer-stabilized ferroelectric liquid crystal or an anti-ferroelectric liquid crystal showing no threshold (voltage) value have been proposed. Although these devices have not been put into practical use sufficiently, it has been reported that a high speed responsiveness on the order of below millisecond is realized.
With respect to the chiral smectic liquid crystal device, our research group has proposed a liquid crystal device as in U.S. patent application Ser. No. 09/338,426 (filed Jun. 23, 1999) (corresponding to Japanese Laid-Open Patent Application (JP-A) 2000-338464) or JP-A 2000-010076 wherein a chiral smectic liquid crystal has a phase transition series on temperature decrease of isotropic liquid phase (Iso)xe2x80x94cholesteric phase (Ch)xe2x80x94chiral smectic C phase (SmC*) or Iso-SmC* and liquid crystal molecules are monostabilized at a position inside an edge of or at an edge position of a virtual cone. During the phase transition of Ch-SmC* or Iso-SmC*, liquid crystal molecular layers are uniformly oriented or aligned in one direction, e.g., by applying a DC voltage of one polarity (+ or xe2x88x92) between a pair of substrates to improve high speed responsiveness and gradation control performance and realize a high luminance liquid crystal device excellent in motion picture image qualities with a high mass productivity. The liquid crystal device of this type may advantageously be used in combination with active elements such as a TFT because the liquid crystal material used has a relatively small spontaneous polarization compared with those used in the conventional chiral smectic liquid crystal devices. The liquid crystal device described in JP-A 2000-010076 can realize a stable gradational (halftone) display with less hysteresis.
As described above, in a sense of solving the problem of conventional nematic liquid crystal devices, i.e., improvement in response speed, the realization of a practical liquid crystal device using a chiral smectic liquid crystal, particularly a monostabilized liquid crystal device as proposed by our research group, is expected for use in advanced displays with high speed responsiveness and good gradation display performance in combination.
In the above-mentioned monostabilized liquid crystal device, however, in order to provide liquid crystal molecules with a uniform layer (formation) direction during an (initial) alignment stage after the liquid crystal is injected into a cell, the liquid crystal has been required to be subjected to a DC voltage application treatment.
As a result, for production of the liquid crystal device, an additional step of cooling the liquid crystal device while applying a DC voltage is required. Further, when the liquid crystal used in the liquid crystal device is once placed in the cholesteric phase under a condition free from application of an external electric field, it is necessary to effect again the DC application treatment. Accordingly, the liquid crystal device is substantially accompanied with a problem such that an upper storage temperature of the liquid crystal device is at most a phase transition temperature (Tc) from Ch (or Iso) to SmC*.
In order to solve the problem, it may be considered that a difference in surface potential is given between a pair of substrates by, e.g., changing a material and/or film structure of opposing surface portions of the pair of substrates, in order to apply a steady-state DC electric field to the liquid crystal.
However, when application of such a steady-state DC electric field is continued in an operational temperature of the liquid crystal device, the liquid crystal device shows an asymmetrical driving characteristic which causes image memory (sticking) phenomenon at the time of long-term drive.
A principal object of the present invention is to provide a chiral smectic liquid crystal device which solves the above-mentioned problems.
A specific object of the present invention is to provide a chiral smectic liquid crystal device free, from an upper limit storage temperature capable of reproducing an alignment state with uniform smectic layer (formation) direction through a cooling operation under no external electric field application even when once placed in cholesteric (or isotropic) phase by ensuring a potential difference between a pair of substrates sufficient to provide a uniform smectic layer (formation) direction in the vicinity of Tc (Ch(or Iso)-SmC* phase transition temperature) under no external electric field application state and a smaller potential difference between the substrates sufficient not to cause driving characteristic deterioration due to image memory etc. in an operation temperature.
Another object of the present invention is to provide a liquid crystal apparatus using the chiral smectic liquid crystal device in combination with drive means for driving the chiral smectic liquid crystal device.
According to the present invention, there is provided a chiral smectic liquid crystal device comprising: a chiral smectic liquid crystal exhibiting a phase transition series on temperature decrease of (a) isotropic liquid phase (Iso), cholesteric phase (Ch) and chiral smectic C phase (SmC*) or (b) isotropic phase (Iso) and chiral smectic C phase (SmC*), and a pair of substrates each provided with an electrode for applying a voltage to the liquid crystal and a uniaxial alignment axis for aligning the liquid crystal, at least one of the substrates being provided with a polarizer and the pair of substrates being oppositely disposed to sandwich the liquid crystal so as to form a plurality of pixels each provided with an active element connected to an associated electrode on at least one of the substrates, wherein the liquid crystal device further includes means for providing a difference in potential between the pair of substrates of at least 100 mV under a condition free from application of an external electric field in a temperature range of Tcxc2x12xc2x0 C. where Tc denotes a phase transition temperature from Ch to SmC* or from Iso to SmC*. Below 100 mV, the resultant layer direction is likely to not be uniform.
In the liquid crystal device, the means may preferably provide a difference in potential between the substrates of at most 100 mV at least in a temperature range of 10-50xc2x0 C. under a condition free from application of an external electric field. Above 100 mV, the image memory due to (asymmetrical) DC bias application is likely to result.
In the above-described cases, the difference in potential in the temperature range of Tcxc2x12xc2x0 C. may preferably be 150 mV to 3 V so as to provide a further uniform layer direction.
In the liquid crystal device of the present invention, the above-mentioned means may preferably comprise a film formed of at least one material providing a difference in potential between the substrates of at least 150 mV at least in a temperature range of 10-75xc2x0 C., more preferably 10-50xc2x0 C., when provided to at least one of the opposing surfaces of the pair of substrates.
According to the present invention, there is also provided a liquid crystal apparatus comprising: a liquid crystal device according to anyone of the above-mentioned liquid crystal devices and drive means for driving the liquid crystal device.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.